1. Field of the Invention
The present invention relates to an error-handling method used in an asynchronous transfer mode (ATM) network, and more particularly to an error-handling method and apparatus used when transmitting over the ATM adaptation layer (AAL) Type 5 the kind of data that cannot be retransmitted.
2. Description of the Related Art
The AAL Type 5 in B-ISDN (Broadband aspects of Integrated Services Digital Network) protocols is being considered, as a simplified AAL protocol, for application to video on demand (VOD), for example.
If the AAL Type 5 is to be applied for transmission of data, such as video data in VOD, that cannot be retransmitted even if received in error at the receiving end, provisions must be made for reliable error handling.
VOD is considered the most promising section in the multimedia field, and trials of VOD have been started in many parts of the worlds, most notably in North America. However, the work to standardize VOD technology is incomplete. Implementing VOD using ATM networks will require equipment such as video servers, ATM switches, and set top boxes (STBs). Furthermore, in reality, in order to achieve fiber to the curb (FTTC), equipment for implementing asymmetrical digital subscriber line (ADSL) technology as well as multiplexing and other equipment will become necessary.
While it is an essential condition to make the interfaces between these pieces of equipment (protocols at each layer) compatible, the work of standardization is still incomplete. Furthermore, these pieces of equipment are large in scale and technically difficult to develop, and it is not possible for any single company to supply all of the equipment; in fact, this is unrealistic. Therefore, collaboration by a plurality of companies will be needed to construct a system, but this is difficult under the present circumstances for the reason stated above.
In a VOD system using an ATM network, image data is run at a higher layer, above the AAL, and the ATM Forum and other organizations are working vigorously to standardize running image data at the higher layer. For image data, standardization of image compression techniques, such as MPEG-1 and MPEG-2, considering the current transmission limits, has been completed by the Moving Picture Coding Experts Group (MPEG) Standardization Committee. Furthermore, an MPEG system for transmitting MPEG-compressed image data on networks has also been standardized, and currently, a method of transmission on ATM networks is being studied by the ATM Forum among others. More recently, a method of mapping MPEG-2 transport stream packets to the AAL Type 5 has been standardized by the ATM Forum as follows:
(1) For ordinary MPEG-2 transport stream packets, two packets are mapped to the AAL Type 5 PDU (protocol data unit) and segmented into eight cells for transmission. In this case, no stuffing is necessary. PA1 (2) If the header of the first packet contains a program clock reference (PCR) as timing clock information, the packet is immediately mapped to the AAL, Type 5 PDU and segmented into five cells for transmission. In this case, stuffing of 44 bytes becomes necessary.
When transmitting MPEG-2 transport stream packets in ATM cells, a phenomenon, called cell delay variation (CDV) or jitter, will become the most serious problem. A large CDV greatly affects the arrival time of PCR which provides the clock recovery information for synchronizing video and audio. For PCR, not only its value but its arrival time is also important, and the above-described method standardized by the ATM Forum alleviates this problem. Nevertheless, occurrence of CDV is unavoidable because of asynchronous transfers, cell discarding, etc. in the ATM network. Therefore, in order to reproduce video data from the packets transmitted from a video server without degrading quality by suppressing occurrence of CDV, a buffer must be provided in intermediate equipment to adjust the arrival time, and also error control must be performed correctly. In error control, if common part convergence sublayer (CPCS) PDUs where errors occurred are simply discarded, a significant effect will be caused in the arrival time of PCR.
To perform the error control correctly, a sequence number should be provided for each CPCS-PDU, but this function is not provided for the AAL Type 5.
To detect loss of MPEG-2 transport stream packets, the continuity counter contained in the header of each packet should be checked, but this is delegated to the higher layer above the AAL. It is, however, difficult to perform error control correctly at the higher layer. Moreover, checking the continuity counter field will lead to complexity of processing since there are two kinds of CPCS-PDUs as described in the above items (1) and (2). It will also become necessary to check the viability of the continuity counter field itself.